Current cellular system implementations involve the use of a few to the use of many cells to cover a given geographical area. The cells are designed to provide some degree of overlapping coverage. They are also designed to allow reuse of the same channels several cells away (but within the same geographical area).
In practice, cellular system cell-site designs do not cover all the desired coverage areas due to anomalies of RF propagation. For example, a narrow depression in the terrain such as a ravine or along a road adjacent to a river bed may not have adequate signal coverage due to blockage from nearby terrain.
Another example would be in an underground parking garage, or even in large office buildings where larger than normal signal attenuation would result in unacceptable signal levels. Furthermore, cell sites in some cellular systems are not located close enough together, thus resulting in poor coverage areas between the cells.
The addition of new cell sites to remedy such problems is prohibitive in many cases. This is because the numbers of subscribers in these areas are generally of insufficient quantity to justify the cost of a new cell site installation.
A low cost alternative solution to this problem is to employ a cellular repeater or booster near the coverage area in question. Such a repeater is intended to retransmit the channels from a nearby (donor) cell into the problem area. The retransmitted channels can then be received by appropriate moveable transceivers or mobile units in the area. Likewise, transmissions from mobile units in the problem area can be retransmitted by the booster such that they can be detected by the channel receivers at the donor cell site. One such booster is disclosed in the commonly assigned patent application hereto, entitled Booster, U.S. Pat. No. 4,941,200 issued Jul. 10, 1991. That patent is hereby incorporated herein by reference.
Cellular systems employ diversity reception of the signals from the mobile to the cell site to overcome the effects of multi-path fading. However, if the signals are amplified by a booster, the multi-path fading occurs on both the path from the mobile to booster and the path from the booster to the cell site. As a result conventional methods of combining the diversity signals may not be effective.
An effective diversity combiner design must take into account the fact that the multi-path fading on each of these two paths has significantly different characteristics. This is because one path length is fixed and the other path length is changing rapidly due to vehicle movement. For example, the fades on the booster to cell path may experience typical fading durations of many seconds in contrast with the mobile to booster fading durations of a few milliseconds when the vehicle is moving.
Switch diversity combining is known. A receiver has two antennas that are separated in space a distance sufficient to uncorrelate the multi-path fading of the signals on the antennas. Only one antenna is connected to the receiver at one time. If the received signal falls below a fixed threshold level, the receiver is switched to the other antenna. This switching algorithm is not optimum for cellular systems boosters.
On the mobile to booster path, the multi-path fading is very rapid when the vehicle is moving, but very slow when the vehicle is stationary. To select the best antenna, the combiner must distinguish between these two conditions. Further, the threshold signal level for switching the antennas should be a function of the received average signal for optimum switching.
On the fixed path between the booster and the cell site or between two boosters, the multi-path fading is very slow and may not change significantly during the duration of one call. However, the signal received by the cell site or booster receiver may vary rapidly because the booster receiving signals from the moveable transceiver has not removed all the multi-path fading caused by the transceiver to booster path. The diversity combiner must ignore these rapid fades and select the antenna having the best average signal level. At the same time the selection must be rapid compared to the duration of a call.